An ink-jet head of ink-jet printers is provided with a less than half the natural period Tc of the ink pressure chambers. This intends to improve ejection efficiency in low-voltage driving.
Further, Japanese Publication for Unexamined Patent Application No. 6-305134 (published on Nov. 1, 1994) discloses a technique that relates to an ink-jet head and a driving method of the inkjet head. This technique teaches that T1, T2≧Tc, and T1, T2≧Ta, where Ta is the period of natural oscillation of piezoelectric elements, Tc is the period of natural oscillation of the ink in the ink pressure chambers, and T2 and T1 are the rise time and fall time, respectively, of the driving voltage of the piezoelectric elements. This is to stabilize the amount of ejected ink and to improve print quality.
The purposes of these prior art documents are all to stabilize the ejection rate. Further, the foregoing publications assume low driving frequencies (on the order of several kilo pulses per second to several tens of kilo pulses per second). These techniques can be applied to a driving mode that uses high driving frequencies (hundreds of kilo pulses per second), such as multi-drop driving, when their rise time T1 and fall time T2 are made shorter. However, a rise time or a fall time that is too short causes the generated heat to accumulate and this increases the head temperature, with the result that ejection characteristics may fluctuate or ejection failure may occur. Another problem is that it increases dissipated power.
On the other hand, when the rise time T2 and fall time T1 are made too long in an effort to lower driving frequencies of the piezoelectric elements, it then becomes necessary to increase the driving voltage and thereby requires a high-voltage power supply to ensure a sufficient amount of ink to be ejected.